Definition
A 75 MHz radio beacon, transmitted from the ground, that marks the final approach fix on the back course of certain ILS approaches. When the aircraft passes over it, the marker beacon receiver in the cockpit produces a distinctive aural tone (a series of two-dot dashes) and illuminates the white marker beacon light.
Plain English
A small ground transmitter on the back side of an ILS approach that gives the pilot a clear in-cockpit signal -- a tone and a white light -- when the aircraft flies over it, telling the pilot they have reached a key point on the approach.
Context Anchor
Seen during instrument approaches that use the back side of a localizer signal, especially in discussions of marker beacons and final approach fixes.
Derivation
"Back course" refers to the signal radiated out the back of an ILS localizer antenna, opposite the normal front-course approach direction. "Marker" comes from the idea that the beacon marks a specific point along the approach path. Together: a marker beacon serving the back-course approach.
Why Pilots Care
Provides positive identification of the final approach fix on the back course, allowing accurate altitude and timing checks without sole reliance on other navigation aids.
Grounding Statement
As the aircraft crosses the marker, the cockpit indication confirms that it has reached a known point on the approach path.
Intuition Check
Do not read “back-course marker” as a marker located behind the airplane. It marks a point on a back-course approach, which uses the reverse side of a localizer signal.
Example Sentence 1
As they crossed the back-course marker, the white light illuminated and the pilot began the final descent to the runway.
Example Sentence 2
Passing the back-course marker confirmed the aircraft position before continuing the non-precision approach.